During the Pennsylvania Ballet's last performance of The Nutcracker last year, there was a curious addition to the delicate "Waltz of the Snowflakes." A dancer dressed as a mouse from the battle scene scurried across the snowy stage, dodging tutus and sending the audience into stitches.

But the surprise cameo was no accident. It was part of a longstanding tradition, the "Nutty Nutcracker," during which the company makes small alterations to its last Nutcracker performance of the season, inspired by inside jokes.

Sometimes the dancers will swap out a prop, or wear a costume from another scene. Often, the orchestra is in on the jokes, as well.

Jermel Johnson, a principal dancer at the ballet, said that earlier in his career, he once wore pointe shoes as a mouse during the battle scene. During the New Year's Eve performance that ushered in 2012, a cellphone went off in the Academy of Music. From the stage, Mother Ginger pantomimed to the offending audience member, Inquirer staff writer  Peter Dobrin reported,  "stretching her pinky and thumb to her lips and ear, mouthing the words 'call me.' "

Some other companies around the county do a Nutty Nutcracker performance, too, and the notoriously picky Balanchine Trust, which keeps an exacting watch over how its namesake's choreography is performed, has been known to laugh along.

"It's exciting to look forward to adding these subtle little things, because by the end, we all know the ballet so well," Johnson said. "Some of the jokes are ones we've carried over from past years, and some of them are new."

How to spot the nuts

That doesn't mean the high jinks are easy to spot, especially because the dancers refrain from altering the story and main choreography. Johnson said if you haven't seen the ballet before, you'll probably miss some of the less obvious ones. The Sugar Plum Fairy may use a wand she made instead of the usual prop. The Mouse King might do a slightly extended, extra-dramatic death scene. Mother Ginger may add jazz hands to her routine.

One year when Johnson danced the candy cane hoop scene, he painted his shoes red and white to match the costume — something that his fellow dancers appreciated, even if the audience didn't notice. He also likes to go through the hoops three times during the last performance instead of the usual two, to the delight of the audience.

"Everyone wants to add something special but subtle to the performance," he said.

Sandra Jennings, who had danced for the New York City Ballet when George Balanchine was alive and who now works for the Balanchine Trust, said the tradition of the Nutty Nutcracker was started by the  choreographer himself, who had a playful sense of humor.

"One year, Balanchine sent one of his dancers for the Arabian scene out with a beard," she said. Jennings once danced choreography from Swan Lake during a stint as lead marzipan.

She said that eventually the New York City Ballet's season-closing performances of The Nutcracker became so famous that people often prepared for the final show by attending an earlier performance to see what "Mr. B had up his sleeve." Eventually, the New York City Ballet stopped performing the "Nutty Nutcracker" because the jokes got so out of hand.

"It was a chance for dancers to let loose a bit," Jennings said. "But it's one of those things where you have to walk a fine line and present a good performance. Balanchine was very clear that we were not allowed to change the story in any way."

‘Nuts’ coast to coast

The Nutty Nutcracker tradition has even spread to ballets that don't perform Balanchine's version. The Boston Ballet, which stages Mikko Nissinen's Nutcracker, has a New Year's Eve performance with unique costume and prop twists. The Pacific Northwest Ballet in Seattle also performed a Nutty Nutcracker on Christmas Eve when it staged the Kent Stowell/Maurice Sendak version of the ballet but stopped when it switched to Balanchine's version in 2015.

"It was very popular with a very specific audience who wanted that kind of irreverence with their holidays," Gary Tucker, spokesman for the Pacific Northwest Ballet, said.

At the Pennsylvania Ballet, ballet master Charles Askegard said dancers come to him for their jokes to be approved before executing them in the final performance. Askegard, a former dancer, once brought his dog on stage for the party scene when he was performing in Los Angeles.

Askegard said that a few years ago, the angels came out with gigantic Starbucks cups instead of little Christmas trees. They did the whole angel dance with the cups and took a sip each time they raised them. The audience adored the little caffeinated angels.

"We've all done the Nutcracker enough," Askegard said. "So the point of this performance is to show our respect for it but also tease it a little bit."